576 research outputs found
Violation of Leggett-Garg inequalities in quantum measurements with variable resolution and back-action
Quantum mechanics violates Leggett-Garg inequalities because the operator
formalism predicts correlations between different spin components that would
correspond to negative joint probabilities for the outcomes of joint
measurements. However, the uncertainty principle ensures that such joint
measurements cannot be implemented without errors. In a sequential measurement
of the spin components, the resolution and back-action errors of the
intermediate measurement can be described by random spin flips acting on an
intrinsic joint probability. If the error rates are known, the intrinsic joint
probability can be reconstructed from the noisy statistics of the actual
measurement outcomes. In this paper, we use the spin-flip model of measurement
errors to analyze experimental data on photon polarization obtained with an
interferometric setup that allows us to vary the measurement strength and hence
the balance between resolution and back-action errors. We confirm that the
intrinsic joint probability obtained from the experimental data is independent
of measurement strength and show that the same violation of the Leggett-Garg
inequality can be obtained for any combination of measurement resolution and
back-action.Comment: 17 pages, 7 figure
Measurement and control of spatial qubits generated by passing photons through double-slits
We present an experimental study of the non-classical correlations of a pair
of spatial qubits formed by passing two down-converted photons through a pair
of double slits. After confirming the entanglement generated in our setup by
quantum tomography using separate measurements of the slit images and the
interference patterns, we show that the complete Hilbert space of the spatial
qubits can be accessed by measurements performed in a single plane between the
image plane and the focal plane of a lens. Specifically, it is possible to
obtain both the which-path and the interference information needed for quantum
tomography in a single scan of the transversal distribution of photon
coincidences. Since this method can easily be extended to multi-dimensional
systems, it may be a valuable tool in the application of spatial qudits to
quantum information processes.Comment: 19 pages, including 10 figures and 2 table
Quantum contextuality of complementary photon polarizations explored by adaptive input state control
We experimentally investigate non-local contextual relations between
complementary photon polarizations by adapting the entanglement and the local
polarizations of a two-photon state to satisfy three deterministic conditions
demonstrating both quantum contextuality and non-locality. The key component of
this adaptive input state control is the variable degree of entanglement of the
photon source. Local polarization rotations can optimize two of the three
correlations, and the variation of the entanglement optimizes the third
correlation. Our results demonstrate that quantum contextuality is based on a
non-trivial trade-off between local complementarity and quantum correlations.Comment: 17 pages, 5 figure
A possible solution to the which-way problem of quantum interference
It is commonly assumed that the observation of an interference pattern is
incompatible with any information about the path taken by a quantum particle.
Here we show that, contrary to this assumption, the experimentally observable
effects of small polarization rotations applied in the slits of a double slit
experiment indicate that individual particles passing the slits before their
detection in the interference pattern are physically delocalized with regard to
their interactions with the local polarization rotations. The rate at which the
polarization is flipped to the orthogonal state is a direct measure of the
fluctuations of the polarization rotation angles experienced by each particle.
Particles detected in the interference maxima experience no fluctuations at
all, indicating a presence of exactly one half of the particle in each slit,
while particles detected close to the minima experience polarization rotations
much larger than the local rotations, indicating a negative presence in one of
the slits and a presence of more than one in the other.Comment: 7 pages, including 1 figure; improved explanation of the relation
with weak values and weak measurements in the introductio
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